In purposing to make an organic polymer adhere to the surface of a metal, glass or the like, it has so far been required to carry out a pretreatment. As for the pretreatment for such a purpose, a so-called "primer treatment" is a particularly effective method, because it is simple in respect of treating process, and excellent in adhesiveness.
In primer compositions for the above-described treatment, various kinds of silane compounds have been employed. Specific examples of such silane compounds include .gamma.-amino-propyltriethoxysilane, N-.beta.-aminoethyl-.gamma.-aminopropyltrimethoxysilane, .gamma.-glycidoxypropyltrimethoxysilane, .gamma.-mercaptopropyltrimethoxysilane, hydrolytic condensation products of these silane compounds, and so on.
In addition, primer compositions using the mixtures produced by the reaction of various kinds of aminosilanes with organic polyisocyanates are disclosed (Japanese Patent Kokoku, examined Japanese patent publication, No. 31550/'89.)
However, those primer compositions are generally slow in curing speed. In particular, the primer compositions utilizing aminosilanes have a defect that they are stained (or colored) by heat drying subsequent to coating.
On the other hand, the product of the reaction between .gamma.-mercaptopropyltrimethoxysilane and tolylenediisocyanate has also been used as a primer composition. This reaction product has a high curing speed and excellent adhesiveness.
However, this primer composition also has a defect that it yellows through staining with a lapse of time after curing. Therefore, it has a problem that it cannot be used as a primer for coating of a coating material of light colors, such as white, pink, etc., e.g., clear lacquer.
As a result of concentrating our energies on solution of the above-described problems, it has now been found that a primer composition containing as a main component the reaction product of a mercaptosilane compound as defined hereinafter with a phenyl group-free organic diisocyanate compound, is only slightly stained after curing, causes no change in color when used as a primer of coating materials of light colors, such as clear lacquer, and can ensure excellent adhesiveness and water-resisting property.
Therefore, an object of this invention is to provide a primer composition which has a high curing speed, enables strong adhesion between the surface of a metal or an inorganic base material and an organic polymer such as paint or lacquer, causes no change in color with a lapse of time, and is well suited to be used for transparent materials, light colored coating materials, and the like.